The invention relates to a process and an apparatus for the filter value and exposure time determination, which is employed in connection with color enlarging apparatus.
In most of the known apparatus employed in color copying work, the technically measurable entity to be determined is the color of the ultimately exposed color paper as a result of the superimposition of the densities in each of the three color layers. Determination of the fileter values adjustably inserted in the enlarging apparatus and exposure time are accomplished by a measurement in the copying plane of the enlarger with a turned-on light source of the intensity of illumination in the individual color components to which the particular color layers of the color paper are sensitive, as well as by the determination of the exposure duration which is necessary in the copying process for the achievement of these densities.
For measuring the intensity of illumination, the output voltage of a light-voltage transducer is compared for each individual color of the three primary color constituents with the help of a zero indicator having a variable voltage source. The value measured is read at the positions of balancing potentiometers which pertain to the individual color components.
In the known apparatus, a total of four measuring channels are present. Three of these measuring channels serve for the evaluation of the individual basic color constituents, wherein in the measuring process, color filters corresponding to these basic colors are inserted into the path of rays of the measuring light, while the fourth channel serves for the determination of the exposure time required in the copying procedure, the respective intensity of illumination being measured by a filter adapted to the spectral properties of the color printing paper. In measuring the exposure time, the amplifying factor of the test amplifier remains constant, while the amplifying factor in the measuring of the individual color components is variable and in its value is unknown. This brings about the considerable disadvantage that the application of such apparatus is limited because the measured values of the color components agree only in relationship to each other and therefore do not allow an absolute determination of the necessary filter values. This disadvantage is contributed to because photomultipliers are used as the light-voltage transducers, and the amplification of photomultipliers must continuously be adapted to the various light conditons in order that the measuring range necessary for practical use may be obtained. These photomultipliers are not linear in their characteristic curve and have the further properties, which are very disadvantageous for use in measuring instruments, of being sensitive to outside light sources, require protection from vibration and, in addition, require very high supply voltages.
As a further disadvantage of the known apparatus, it has been shown that they require constant attention during operation in order that errors may be balanced out which occur in the receiver and in the measuring circuit due to temperature and to aging. In the servicing of these apparatus, it is also disadvantageous that for the null reading in the balancing procedure and the exposure reading, measuring instruments are employed, which very easily lead to reading errors due to parallax, because operator is constantly situated in different positions relative to the measuring instrument in the various adjusting operations on the enlarger, the equalizing procedure additionally being made more difficult by the swinging of the indicator. Also, the reading of the exposure time takes place by means of the same measuring instrument with the help of a logarithmic scale, which reproduces only a part of the exposure time range needed in practice sufficiently well and, in addition, does not allow an exact evaluation of intermediate values.
The object of the invention is to develop an apparatus for the filter and exposure time determination which does not have the stated disadvantages, wherein the measuring values of the color components are not only determined in relationship to each other but wherein a measurement of the absolute values is possible; furthermore, wherein no further calibration is necessary and an exposure time determination corresponding to the spectral properties of the photographic color printing paper is possible without a special measuring channel being required with its own filter, equalizing potentiometer and indicating instrument for the indication of the exposure time.